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The access to the contents of this doctoral thesis it is limited to the acceptance of the use conditions set by the following Creative Commons license: https://creativecommons.org/licenses/?lang=en Doctoral Thesis MAGIC observations with bright Moon and their application to measuring the VHE gamma-ray spectral cut-off of the PeVatron candidate Cassiopeia A Daniel Alberto Guberman Departament de F´ısica Universitat Aut`onomade Barcelona Programa de doctorat en F´ısica Director: Director: Tutor: Dr. Abelardo Dr. Juan Cortina Dr. Enrique Moralejo Olaizola Blanco Fern´andezS´anchez A thesis submitted for the degree of PhilosophiæDoctor Submitted in 2018 Director: Director: Dr. Abelardo Moralejo Olaizola Dr. Juan Cortina Blanco IFAE IFAE Edifici Cn, UAB Edifici Cn, UAB 08193 Bellaterra (Barcelona) Spain 08193 Bellaterra (Barcelona) Spain [email protected] [email protected] Tutor: Dr. Enrique Fern´andezS´anchez IFAE & UAB Edifici Cn, UAB 08193 Bellaterra (Barcelona) Spain [email protected] I herewith declare that I have produced this thesis without the prohibited assistance of third parties and without making use of aids other than those specified; notions taken over directly or indirectly from other sources have been identified as such. This paper has not previously been presented in identical or similar form to any other Spanish or foreign examination board. The thesis work was conducted from January 2014 to September 2018 under the supervision of Abelardo Moralejo and Juan Cortina at Barcelona. Barcelona, June 28, 2018. Contents Introduction 7 1 Cosmic Rays 9 1.1 Galactic and extragalactic Cosmic Rays . 9 1.2 The puzzle of the galactic cosmic-ray origin . 11 1.3 Cosmic-ray electrons . 12 2 Searching for sources of cosmic rays 15 2.1 Direct and indirect detection of cosmic rays . 15 2.2 Signatures of cosmic ray sources in gamma rays . 17 2.2.1 Gamma-ray production from cosmic rays and electrons . 18 2.3 Gamma-ray detection techniques . 21 2.3.1 Gamma-ray direct detection with Fermi-LAT . 23 2.3.2 The Imaging Atmospheric Cherenkov Technique . 24 2.4 Signatures of cosmic-ray sources in neutrinos . 26 3 SNRs as cosmic ray accelerators 29 3.1 Diffusive Shock Acceleration . 31 3.2 Evidence of particle acceleration in SNRs . 33 3.3 Cassiopeia A . 34 3.4 Alternatives to the SNR hypothesis . 37 4 Adapting MAGIC for moonlight observations 39 4.1 The MAGIC telescopes . 39 4.1.1 The MAGIC cameras . 41 4.1.2 Readout and trigger system . 41 4.1.3 Data taking . 44 4.1.4 Analysis chain . 45 4.1.5 Monte Carlo Simulations . 53 4.1.6 High level analysis products . 55 4.2 Moonlight observations . 61 4.2.1 Hardware settings for moonlight observations . 62 4.2.2 UV-pass Filters . 64 4.2.3 Data sample . 70 4.2.4 Moonlight adapted analysis . 71 4.2.5 Performance . 75 4.2.6 Towards increasing the duty cycle . 88 5 6 CONTENTS 5 Cosmic-ray acceleration in Cas A 91 5.1 Cas A observations . 91 5.1.1 Observations with MAGIC . 91 5.1.2 Observations with Fermi-LAT . 93 5.2 Results . 93 5.2.1 Spectrum . 93 5.3 Statistical and systematic uncertainties . 96 5.3.1 Individual spectra . 96 5.3.2 Statistical and systematic uncertainties in the spectrum . 97 5.3.3 Fit Results . 100 5.3.4 Safety check with Crab Nebula samples . 100 5.4 Interpretation . 105 5.4.1 Leptonic model . 105 5.4.2 Hadronic model . 106 Conclusions 109 A Performance under moonlight: auxiliary analysis 113 A.1 Hillas distributions . 113 A.2 15-30 ×NSBDark UV-pass filter spectrum . 113 B The Light-Trap detector 121 B.1 Silicon photomultipliers: a new era begins? . 121 B.2 Silicon photomultipliers: a brief technical review . 122 B.2.1 Operational principle . 122 B.2.2 Pulse shape . 123 B.2.3 Over-voltage, gain and PDE . 126 B.2.4 Noise . 127 B.2.5 Temperature dependence . 130 B.3 The Light-Trap pixel principles . 130 B.4 Proof-of-Concept Pixel . 131 B.4.1 SiPM . 131 B.4.2 Disc . 131 B.4.3 Optical coupling . 135 B.4.4 Reflective walls . 135 B.4.5 Pixel holder and readout electronics . 135 B.4.6 Laboratory measurements . 135 C Moon shadow observations with MAGIC 145 C.1 Feasibility study . 146 C.1.1 Energy range and camera acceptance . 147 C.1.2 Background and electron rates . 147 C.1.3 Observation time needed to detect the electron shadow . 150 D The naima package 153 Introduction In 1962 Bruno Rossi wrote the last pages of a book in which he described how high- energy astrophysics emerged and developed over the 50 years that followed what we now consider as its birth. This book was called Cosmic Rays [174] and commenced as follows: \A steady rain of charged particles, moving at nearly the speed of light, falls upon our planet at all times and from all directions." More than fifty years after Rossi's book high-energy astrophysics has evolved into a very mature and productive field. But still nowadays many of the questions that were asked by that time still remain unanswered. The origin of this steady rain of charged particles, the cosmic rays, is probably one of the biggest mysteries that we still have not been able to solve. For years we hoped, an we still do, that gamma-ray instruments would point us to the source in which the bulk of these cosmic rays acquire their energy. And for a long time it was believed, and it is still believed, that young supernova remnants would be those sources. But even if we have found evidence of particle acceleration in these systems we still could not find one remnant that is capable of accelerating protons up to energies high enough to explain the observed cosmic-ray flux. This thesis aims to shed some light on this matter. In the following pages I will describe a study on particle acceleration in the young supernova remnant Cassiopeia A, one of the most promising candidates to explain the origin of galactic cosmic rays. It reports on my work during the last four years, based on observations performed with the MAGIC gamma-ray telescopes, which I performed with the invaluable assistance of many colleagues. My will is also that by the end of the thesis I could transmit the message that behind a main result there is a technical development that played an essential role. And of course, to acknowledge the work of many others, in the theoretical and the experimental side, that motivated the observations and the analysis I will describe. In Chapter 1 I briefly introduce the problem of the origin of cosmic rays. Chapter 2 describes different experimental approaches to detect cosmic rays and to try to identify their potential sources. Chapter 3 is devoted to supernova remnants, the most popular candidates to be the sources that accelerate galactic cosmic rays up to the highest observed energies. I will focus on Cassiopeia A, the observational target of this thesis. In Chapter 4 I describe the instrument I used to perform the observations, the MAGIC telescopes. Our strategy to accumulate enough observation time to have good statistics was to extend the telescopes duty cycle by operating them under moonlight. I will then also describe all the hardware and software modifications I had to perform to safely 7 8 INTRODUCTION operate the telescopes under such conditions, analyze the data taken and evaluate their performance. Finally, in Chapter 5 I present the observation campaign on Cassiopeia A, the results obtained and their interpretation. Appendix B hosts the work I did in the development and characterization of a new concept of pixel detector, initially meant for Cherenkov astronomy, but with potential applications beyond the field. I decided it to keep it outside the main body of the text because it does not directly fit in the discussion of the cosmic-ray origin. But since I devoted a considerable amount of time to this task and I consider it to be an essential part of my formation I thought it deserved its place in this thesis. Chapter 1 Cosmic Rays The history of cosmic-ray astrophysics can be traced back to 1912. Aiming to explain a mysterious intrinsic radiation that was observed in electrometers, Victor Hess per- formed seven balloon flights to measure the radiation in the atmosphere at different heights. He found that the radiation increased at high altitudes (above 2000 m), from which he concluded that it could not be originated in the Earth as he initially thought. In his article he wrote: \The results of the present observations seem to be most readily explained by the assumption that a radiation of very high penetrating power enters our atmosphere from above"[93]. By that time the most penetrating radiation known were gamma rays from radioactive materials. Then it was natural to think that this new unknown radiation could also be of electromagnetic origin. Robert Millikan then called this radiation cosmic rays [174]. Nowadays we know that cosmic rays are composed of heavy charged particles orig- inated outside the solar system. Most of them are protons (∼90%), about 9% are helium nuclei and the remaining fraction is composed by heavier nuclei.

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